gallium/{r600, radeonsi}: Fix segfault with color format (v2)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102552



v2: Patch cleanup proposed by Nicolai Hähnle.
    * deleted changes in si_translate_texformat.

Cc: Nicolai Hähnle <nhaehnle@gmail.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 74d24564)